Polyimide copolymer and metal laminate using the same

ABSTRACT

A novel polyimide copolymer, which is a copolymer comprising two kinds of tetracarboxylic acid dianhydrides consisting of (A) isopropylidene-bis(4-phenyleneoxy-4-phthalic acid) dianhydride and (B) 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride, and one kind of a diamine consisting of (C) 6-amino-2-(p-aminophenyl)benzimidazole, or two or three kinds of diamines consisting of component (C) and (D) at least one kind of diamines consisting of bis(4-amino-phenyl)ether (D 1 ) and phenylenediamine (D 2 ), and a metal laminate manufactured by laminating said polyimide copolymer to a metallic foil. The metal laminate comprising the novel polyimide copolymer as a layer on the metallic foil has a low curling susceptibility to cause curling, twisting, warping, etc. against temperature changes due to a low coefficient of linear thermal expansion of the polyimide copolymer, and also has satisfactory adhesiveness and thermal dimensional stability and a low water absorbability, and thus can be used as a suitable for a flexible, finely printed circuit board requiring a high dimensional stability.

TECHNICAL FIELD

The present invention relates to a novel polyimide copolymer and a metallaminate using the same, and more particularly to a novel polyimidecopolymer capable of suitably using as a film for a flexible printedcircuit board and a metal laminate using the same.

BACKGROUND ART

Most of substrates for flexible printed circuit boards, etc. have beenso far manufactured by bonding a metallic foil and an aromatic polyimidefilm with an adhesive such as epoxy resin, polyurethane resin, etc.However, the flexible printed circuit boards manufactured with such anadhesive have problems such as adhesive peeling due to successivethermal compression bonding hysteresis or due to exposure to elevatedtemperatures in the soldering step, or smear generation in the drillingstep owing to an adhesive, and further have such drawbacks as curling,twisting, warping, etc. of the substrates after cooling, causing troubleparticularly to form a fine pattern.

These problems owe their origin to differences in coefficient of linearthermal expansion between a conductor and an insulating material, andthus it has been proposed to improve the heat resistance of theadhesive. Basically, omission of the adhesive layer can solve not onlythe problems due to the adhesive layer, but can also save the labor ofbonding the adhesive to the polyimide film.

From such a viewpoint, a metal laminate has been formed in some cases bydirectly coating a metallic conductor with polyamic acid as a polyimideprecursor copolymer, followed by heating to effect polyimidization. Itis known that the metal laminate thus obtained has a poor dimensionalstability and suffers from curling.

To overcome these drawbacks, JP-B-5-22399, JP-B-6-93537, JP-B-7-39161,etc. disclose metal laminates with distinguished dimensional stability,adhesiveness, flatness after etching, reduction in curling, etc.,manufactured by forming a plurality layers of a polyimide resin layerhaving low thermal expansion and other polyimide resin layers on aconductor, where two or three kinds of polyimide precursor copolymersmust be used, the individual copolymer solutions must be applied one byone to the conductor to form an insulation multilayer, and a ratio inthickness of the resulting individual polyimide layers must bespecified, inevitably complicating the manufacture of the metal laminatethereby.

The present applicant has already proposed a metal laminate where onekind of polyimide copolymer layer is directly laminated to a metallicconductor (WO 01/29136). The polyimide copolymer used therein is acopolymer of isopropylidenebis(4-phenyleneoxy-4-phthalic acid)dianhydride and 6-amino-2-(p-aminophenyl)benzimidazole. The polyimidecopolymer obtained by polycondensation of these monomer components has ahigh adhesive strength by itself and can give a metal laminate with asatisfactory peel strength even if laminated directly to a metallic foilwithout interposing an adhesive layer therebetween, and also has a goodsolder heat resistance, but originally the polyimide copolymer has notbeen intended to lower the coefficient of linear thermal expansion orpercent heat shrinkage or improvement of curling resistance.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a polyimide copolymercapable of giving a flexible printed circuit board with satisfactoryadhesive strength and dimensional stability in a metal laminatecomprising one kind of polyimide copolymer layer formed on the metallicconductor, by lowering the coefficient of linear thermal expansion orpercent heat shrinkage of the polyimide copolymer or approaching thecoefficient of linear thermal expansion or percent heat shrinkage of thepolyimide copolymer to that of the conductor, thereby effectivelysuppressing curling, twisting, warping, etc. of the metal laminate evenif subjected to heat hysteresis, and also to provide a metal laminateusing the same.

The object of the present invention can be attained by a novel polyimidecopolymer, which is a copolymer comprising two kinds of tetracarboxylicacid dianhydrides consisting of (A)isopropylidenebis(4-phenyleneoxy-4-phthalic acid) dianhydride and (B)3,3′,4,4′-biphenyltetracarboxylic acid dianhydride, and one kind of adiamine consisting of (C) 6-amino-2-(p-aminophenyl)benzimidazole or twoor three kinds of diamines consisting of said component (C) and (D) atleast one kind of diamines consisting of bis(4-aminophenyl)ether (D₁)and phenylenediamine (D₂), and also by a metal laminate comprising ametallic foil and a layer of said polyimide copolymers laminatedthereto. The present novel polyimide copolymer is film-formable.

Tetracarboxylic acid dianhydrides for use in the synthesis of thepresent novel polyimide copolymer are two kinds of acid dianhydridesconsisting of (A) isopropylidenebis(4-phenylene-oxy-4-phthalic acid)dianhydride as shown below:

and (B) 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride as shownbelow:

Component (A) and component (B) are used in a proportion of component(A) to component (B) of 10-80% by mole, preferably 20-60% by mole to90-20% by mole, preferably 80-40% by mole. When a proportion ofcomponent (A) is more than 80% by mole, curling will be pronounced inthe stage of forming a metal laminate, together with an increasedcoefficient of linear thermal expansion and also with an increasedpercent heat shrinkage. Whereas when a proportion of component (A) isless than 10% by mole, the resulting film will become not only brittle,but also the adhesive strength will be no more observable in the metallaminate.

Other kinds of tetracarboxylic acid dianhydrides can be used togetherwithin such a range as not to deteriorate the object of the presentinvention.

Diamine for use in forming a polyimide copolymer upon reaction withthese two kinds of tetracarboxylic acid dianhydrides is (C)6-amino-2-(p-aminophenyl) benzimidazole as shown below:

The above mentioned diamine compound (C) can be used together with (D)at least one of bis(4-aminophenyl)ether (D₁) and phenylenediamine, forexample, p-phenylenediamine (D₂) as shown below:

When bis(4-aminophenyl)ether (D₁) is used as diamine compound (D), aproportion of component (D₁) to component (C) is not more than 40% bymole, preferably 30-10% by mole, to not less than 60% by mole,preferably 70-90% by mole. When a proportion of component (D₁) tocomponent (C) is more than 40% by mole, curling will be pronounced inthe stage of forming a metal laminate, together with an increasedcoefficient of linear thermal expansion.

When phenylenediamine (D₂) is used as diamine compound (D), a proportionof component (D₂) to component (C) is not more than 80% by mole,preferably 70-50% by mole to not less than 20% by mole, preferably30-50% by mole. When a proportion of component (D₂) to component (C) ismore than 80% by mole, curling will be pronounced in the stage offorming a metal laminate.

When bis(4-aminophenyl)ether (D₁) and phenylenediamine (D₂) are usedtogether as diamine compounds (D), a proportion of sum total ofcomponents (D₁) and (D₂) to component (C) is generally not more than 75%by mole to not less than 25% by mole, though dependent on a proportionof diamine compound (D₁) to diamine compound (D₂), and component (C) andcomponent (D) can be used in such a molar ratio as not to cause curlingin the stage of forming a metal laminate.

Other kinds of diamine compounds can be used together in such a range asnot to deteriorate the object of the present invention.

Reaction of tetracarboxylic acid dianhydrides with diamine(s) is carriedout preferably in a N-methyl-2-pyrrolidone solvent, but can be carriedout also in a polar solvent such as dimethylformamide,dimethyl-acetamide, m-cresol, etc. Actually, diamine (mixture) or itssolution in a polar solvent is dropwise added to a solution in a polarsolvent of tetracarboxylic acid dianhydride mixture at about 0°-about60° C., and then subjected to reaction at about 0-about 60° C. for about0.5-about 5 hours to form polyamic acid as a polyimide precursorcopolymer.

The polyamic acid solution in a polar solvent is applied to a metallicfoil, typically a copper foil, and after removal of the solvent bydrying, polyimidization reaction is carried out by heating. To promotedehydration cyclization reaction for the polyimidization, the polyamicacid-applied metallic foil is passed through a drying oven heated to atemperature of about 150°-about 450° C., preferably about 200° C.-about400° C., to form a metal laminate substantially free from the solvent.The polar solvent used in the synthesis reaction of polyimide precursorcopolymer can be used as a polar solvent for the polyamic acid as such.N-methyl-2-pyrrolidone is a preferable polar solvent.

As a result of reaction between tetracarboxylic acid dianhydride (A) anddiamine (C), a polyimide copolymer with the following repeat unit can beobtained:

As a result of the reaction between the tetracarboxylic acid dianhydride(B) and the diamine (C), a polyimide copolymer with the following repeatunit can be obtained in addition to the above-mentioned repeat unit:

When diamine (D₁) is used together with diamine (C), a polyimidecopolymer with the following repeat units can be obtained by reactionwith the tetracarboxylic acid dianhydrides (A) and (B):

When diamine (D₂) is used together with diamine (C), a polyimidecopolymer with the following repeat units can be obtained by reactionwith the tetracarboxylic acid dianhydrides (A) and (B):

Polyimide copolymers having said repeat units are insoluble in varioussolvents and thus their molecular weights or viscosities cannot bedetermined or the ranges thereof cannot be specified, but it is certainthat these polyimide copolymers have molecular weights necessary forenabling film formation. In the stage of polyamic acids which are deemedto be in a form of polyimide precursor copolymers, it is possible todetermine a viscosity of a reaction mixture solution at a givenconcentration of solid matters, but the viscosity is of variable nature,e.g. dependent on reaction time, etc.

Polyimide copolymer obtained by polycondensation ofisopropylidene-bis(4-phenyleneoxy-4-phthalic acid) dianhydride and6-amino-2-(p-aminophenyl)benzimidazole, as disclosed in theafore-mentioned WO 01/29136, is soluble by itself in a solvent such asdimethyl formamide, dimethyl acetamide, N-methyl-2-pyrrolidone,m-cresol, etc., and when a solution of such a copolymer is applied to ametallic foil, a metal laminate with a satisfactory peel strength can beobtained.

On the other hand, the present polyimide copolymer obtained by usingisopropylidenebis(4-phenyleneoxy-4-phthalic acid) dianhydride and3,3′,4,4′-biphenyltetracarboxylic acid dianhydride as acid dianhydridesto be polycondensated with 6-amino-2-(p-aminophenyl)benzimidazole isinsoluble in various solvents, as mentioned above, and a metal laminateobtained by laminating the present polyimide copolymer to a metallicfoil has a distinguished curling resistance and an improved coefficientof linear thermal expansion or percent heat shrinkage.

A polyimide copolymer film having a thickness of about 5-about 50 μmwith good mechanical strength and thermal dimensional stability can bealso obtained by removing the metallic foil from the thus formed metallaminate.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention will be described below, referring to Examples.

EXAMPLE 1

A solution containing 520.1 g (1.0 mole) of (A)isopropylidenebis(4-phenyleneoxy-4-phthalic acid) dianhydride and 294.0g (1.0 mole) of (B) 3,3′,4,4′-biphenyltetracarboxylic acid dianhydridein 7,150 ml of N-methyl-2-pyrrolidone was charged into a four-neckedflask having a capacity of 10 L and provided with a stirrer in anitrogen gas-flushed atmosphere, and then 448.0 g (2.0 moles) of (C)6-amino-2-(p-aminophenyl)-benzimidazole was charged thereto, whilekeeping the temperature not higher than 60° C. The resulting mixture wasstirred at room temperature for three hours to obtain 8,245 g of avarnish-state polyimide precursor copolymer solution (viscosity at 25°C.: 7,900 cps; concentration of solid matters: 15 wt. %).

The above-mentioned polyimide precursor copolymer solution was appliedto a roughened surface of rolled electrolytic copper foil (product ofFurukawa Electric Co., Ltd.; thickness: 10 μm) with a coat thickness of18 μm using a reverse type roll coater, and then the solvent wascontinuously removed therefrom through a hot air drying oven at 120° C.,followed by heat treatment by elevating the temperature up to 400° C.over 10 minutes to form a 12.5 μm-thick polyimide layer on the copperfoil.

EXAMPLE 2

In Example 1, the amount of component (A) was changed to 260.0 g (0.5moles), that of component (B) to 441.0 g (1.5 moles) and that ofN-methyl-2-pyrrolidone to 6,510 ml, respectively, and 7,572 g of avarnish-state polyimide precursor copolymer solution (8,200 cps; 15 wt.%) was obtained. A polyimide-laminated copper foil was manufactured fromthe thus obtained polyimide precursor copolymer solution in the samemanner as in Example 1.

EXAMPLE 3

In Example 2, the amount of N-methyl-2-pyrrolidone was changed to 6,520ml and that of component (C) to 403.2 g (1.8 moles), respectively, while40.0 g (0.2 moles) of (D₁) bis(4-aminophenyl)ether was additional used.7,470 g of a varnish-state polyimide precursor copolymer solution (5,500cps; 15 wt. %) was obtained. A polyimide-laminated copper foil wasmanufactured from the thus obtained polyimide precursor copolymersolution in the same manner as in Example 1.

EXAMPLE 4

In Example 3, the amount of N-methyl-2-pyrrolidone was changed to 6,200ml, that of component (C) to 313.6 g (1.4 moles) and that of component(D₁) to 80.0 g (0.6 moles), respectively. 7,221 g of a varnish-statepolyimide precursor copolymer solution (3,800 cps; 15 wt. %) wasobtained. A polyimide-laminated copper foil was manufactured from thethus obtained polyimide precursor copolymer solution in the same manneras in Example 1.

The copper foil/polyimide laminates obtained in the foregoing Examplesand polyimide films obtained by removing the copper foils from thelaminates by etching were subjected to determination of the followingitems.

Glass transition temperature (Tg): Loss elastic modulus E″ in terms ofPa unit was obtained from dynamic viscoelasticity determined by adynamic viscoelasticity analyzer DMe 7e made by Parkin Elmer Co., Ltd.and maximum E″ was made “Tg”

Coefficient of linear thermal expansion (100°-200° C.) (CTE): A filmsample obtained from a laminate, 10 cm×10 cm, by etching, and stressrelaxed by heating to 400° C. was fixed to a TMA tester and subjected todetermination in a tensile mode under such conditions as load: 2 g,sample length: 20 mm and temperature elevation rate: 10° C./min

Adhesive strength: A laminate, 1 cm×10 cm, was subjected todetermination according to JIS C-6481

Tensile strength and elongation at break: A film obtained from alaminate, 10 cm×20 cm, by etching was subjected to determinationaccording to ASTM D-882-83

Water absorbability: A film obtained from a laminate, 11 cm×11 cm, byetching was dried at 150° C. for 60 minutes (dry weight W₁) and thendipped in distilled water at 23° C. for 24 hours (after-dipping weightW₂), then determined as a change in weight by the equation of (W₂−W₁)W₁100

Percent shrinkage after etching: Percent dimensional changes in MDdirection and TD direction before and after etching were determinedaccording to JIS C-6481

Percent heat shrinkage: A film obtained from a laminate, 10 cm×20 cm, byetching was subjected to heat treatment in a hot air oven at 150° C. for30 minutes to determine percent dimensional changes in MD direction andTD direction before and after the heat treatment

Curling: A laminate, 5 cm×5 cm, was gently placed on a horizontal flatbase to bring the laminate into a concave state thereon, and the statewas visually observed without applying any particular external forcethereto

Results of determination and observation are shown in the followingTable 1.

TABLE 1 Determination/observation item Ex. 1 Ex. 2 Ex. 3 Ex. 4 Tg (° C.)306 323 318 301 CTE (ppm/° C.) 32 17 23 32 Adhesive strength (Kg/cm)1.80 1.92 1.85 1.90 Tensile strength (MPa) 164 244 237 240 Elongation atbreak (%) 46 36 47 66 Water absorbability (%) 2.98 3.36 3.05 2.27Percent shrinkage after etching MD direction (%) −0.05 0.099 0.067−0.054 TD direction (%) −0.118 0.061 0.048 −0.084 Percent heat shrinkageMD direction (%) −0.165 0.121 0.037 −0.086 TD direction (%) −0.181 0.050.013 −0.107 Curling Laminate flat flat flat flat

COMPARATIVE EXAMPLE 1

In Example 1, the amount of component (B) was changed to 588.0 g (2.0moles) and that of N-methyl-2-pyrrolidone to 5,870 ml, respectively,without using component (A). 6,699 g of a varnish-state polyimideprecursor copolymer solution (6,450 cps; 15 wt. %) was obtained. Apolyimide-laminated copper foil was manufactured from the thus obtainedpolyimide precursor copolymer solution in the same manner as in Example1.

COMPARATIVE EXAMPLE 2

In Example 1, the amount of component (A) was changed to 936.0 g (1.8moles), that of component (B) to 58.8 g (0.2 moles) and that ofN-methyl-2-pyrrolidone to 8,170 ml, respectively. 9,234 g of avarnish-state polyimide precursor copolymer solution (2,500 cps; 15 wt.%) was obtained. A polyimide-laminated copper foil was manufactured fromthe thus obtained polyimide precursor copolymer solution in the samemanner as in Example 1.

COMPARATIVE EXAMPLE 3

In Example 3, the amount of N-methyl-2-pyrrolidone was changed to 6,400ml, that of component (C) to 224.0 g (1.0 mole) and that of component(D₁) to 200.0 g (1.0 mole), respectively. 7,375 g of a varnish-statepolyimide precursor copolymer solution (2,700 cps; 15 wt. %) wasobtained. A polyimide-laminated copper foil was manufactured from thethus obtained polyimide precursor copolymer solution in the same manneras in Example 1.

COMPARATIVE EXAMPLE 4

In Example 3, the amount of N-methyl-2-pyrrolidone was changed to 6,270ml, that of component (C) to 44.8 g (0.2 moles) and that of component(D₁) to 360.0 g (1.8 moles), respectively. 6,981 g of a varnish-statepolyimide precursor copolymer solution (7,900 cps; 15 wt. %) wasobtained. A polyimide-laminated copper foil was manufactured from thethus obtained polyimide precursor copolymer solution in the same manneras in Example 1.

The copper foil/polyimide laminates obtained in the foregoingComparative Examples and polyimide films obtained by removing the copperfoils from the laminates by etching were subjected to the samedetermination and observation as in Examples 1 to 4. Results ofdetermination and observation are shown in the following Table 2. InComparative Example 1, the film after etching was so brittle thatdetermination of “percent shrinkage after etching” and “percent heatshrinkage” could not be determined.

TABLE 2 Comp. Comp. Comp. Comp. Determination/observation item Ex. 1 Ex.2 Ex. 3 Ex. 4 Tg (° C.) 341 301 276 266 CTE (ppm/° C.) 6.5 48 39 47Adhesive strength (Kg/cm) 0.5 — — — Tensile strength (MPa) 329 133 191185 Elongation at break (%) 20 60 62 80 Water absorbability (%) 3.752.61 1.93 1.20 Percent shrinkage after etching MD direction (%) — −0.163−0.136 −0.285 TD direction (%) — −0.228 −0.242 −0.270 Percent heatshrinkage MD direction (%) — −0.404 −0.237 −0.401 TD direction (%) —−0.411 −0.346 −0.495 Curling Laminate large large large large

EXAMPLE 5

A solution containing 208.0 g (0.4 moles) of (A)isopropylidenebis(4-phenyleneoxy-4-phthalic acid) dianhydride and 470.4g (1.6 moles) of (B) 3,3′,4,4′-biphenyltetracarboxylic acid dianhydridein 5,460 ml of N-methyl-2-pyrrolidone was charged into a four-neckedflask having a capacity of 10 L and provided with a stirrer in anitrogen gas-flushed atmosphere, and a mixture consisting of 134.4 g(0.6 moles) of (C) 6-amino-2-(p-aminophenyl) benzimidazole and 152.2 g(1.4 moles) of (D₂) p-phenylenediamine was added thereto, while keepingthe temperature not higher than 30° C. Then, the resulting mixture wasstirred at room temperature for three hours to obtain 6,420 g of apolyimide precursor copolymer solution (concentration of solid matters:15 wt. %; viscosity at 25° C.: 2,150 cps).

The resulting polyimide precursor varnish was continuously applied tothe roughened surface of a rolled electrolytic copper foil (thickness:10 μm; a product made by the Furukawa Electric Co., Ltd.) with a coatthickness of 18 μm using a reverse type roll coater, and then thesolvent was continuously removed therefrom through a hot air drying ovenat 120° C., followed by heat treatment for polyimidization by elevatingthe temperature up to 400° C. over 10 minutes to obtain a copperfoil/polyimide laminate with a 12.5 μm-thick polyimide layer and withoutany curling.

EXAMPLE 6

In Example 5, the amount of N-methyl-2-pyrrolidone was changed to 5,730ml, that of component (C) to 224.0 g (1.0 mole) and that of component(D₂) to 108.0 g (1.0 mole), respectively. 6,736 g of a polyimideprecursor copolymer solution (concentration of solid matters: 15 wt. %;viscosity: 1,850 cps) was obtained. A curling-free copper foil/polyimidelaminate was also manufactured from the thus obtained polyimideprecursor varnish in the same manner as in Example 5.

EXAMPLE 7

In Example 5, the amount of component (A) was changed to 260.0 g (0.5moles), that of component (B) to 441.0 g (1.5 moles), that ofN-methyl-2-pyrrolidone to 5,720 ml, that of component (C) to 179.2 g(0.8 moles) and that of component (D₂) to 129.2 g (1.2 moles),respectively. 6,723 g of a polyimide precursor copolymer solution(concentration of solid matters: 15 wt. %; viscosity: 2,180 cps) wasobtained. A curling-free copper foil/polyimide laminate was alsomanufactured from the thus obtained polyimide precursor varnish in thesame manner as in Example 5.

EXAMPLE 8

In Example 5, the amount of component (A) was changed to 260.0 g (0.5moles), that of component (B) to 441.0 g (1.5 moles), that ofN-methyl-2-pyrrolidone to 5,850 ml, that of component (C) to 224.0 g(1.0 mole) and that of component (D₂) to 108.0 g (1.0 mole),respectively. 6,880 g of a polyimide precursor copolymer solution(concentration of solid matters: 15 wt. % viscosity: 2,300 cps) wasobtained. A curling-free copper foil/polyimide laminate was alsomanufactured from the thus obtained polyimide precursor varnish in thesame manner as in Example 5.

EXAMPLE 9

A solution containing 156.0 g (0.3 moles) of (A)isopropylidenebis(4-phenyleneoxy-4-phthalic acid) dianhydride and 500.0g (1.7 moles) of (B) 3,3′,4,4′-biphenyltetracarboxylic acid dianhydridein 5,800 ml of N-methyl-2-pyrrolidone was charged into a four-neckedflask having a capacity of 10 L and provided with a stirrer in anitrogen gas-flushed atmosphere, in a mixture consisting of 180.0 g (0.8moles) of (C) 6-amino-2-(p-aminophenyl) benzimidazole, 120.0 g (0.6moles) of (D₁) bis(4-aminophenyl)ether and 64.0 g (0.6 moles) of (D₂)p-phenylenediamine was added thereto, while keeping the temperature nothigher than 30° C. The mixture was stirred at room temperature for threehours to obtain 6,800 g of a polyimide precursor copolymer solution(concentration of solid matters: 15 wt. %: viscosity at 25° C.: 5,500cps). A curling-free copper foil/polyimide laminate was alsomanufactured from the thus obtained polyimide precursor varnish in thesame manner as in Example 5.

COMPARATIVE EXAMPLE 5

In Example 5, the amount of component (B) was changed to 588.0 g (2.0moles), that of N-methyl-2-pyrrolidone to 5,260 ml, that of component(C) to 134.4 g (0.6 moles) and that of component (D₂) to 86.4 g (0.8moles), respectively, without using component (A). 120.0 g (0.6 moles)of bis(4-aminophenyl)ether was further used. 6,182 g of a polyimideprecursor copolymer solution (concentration of solid matters: 15 wt. %;viscosity 3,200 cps) was obtained. A copper foil/polyimide laminate wasalso manufactured from the thus obtained polyimide precursor varnish inthe same manner as in Example 5.

COMPARATIVE EXAMPLE 6

In Example 5, the amount of N-methyl-2-pyrrolidone was changed to 5,060ml and that of component (D₂) to 216.0 g (2.0 moles), respectively,without using component (C). 5,963 g of a polyimide precursor copolymersolution (concentration of solid matters: 15 wt. %; viscosity: 1,000cps) was obtained. A copper foil/polyimide laminate was alsomanufactured from the thus obtained polyimide precursor varnish in thesame manner as in Example 5.

The copper foil/polyimide laminates obtained in the foregoing Examples 5to 9 and Comparative Example 5 and polyimide films obtained by removingthe copper foils from the laminates by etching were subjected to thesame determination and observation as in Examples 1 to 4. Determinationof elastic modulus and evaluation of curling were carried out in thefollowing manner:

Elastic modulus: Films obtained from laminates, 10 cm×20 cm, by etchingwere subjected to determination according to ASTM D-882-83

Curling: Laminates, 5 cm×5 cm, films obtained therefrom by etching andthe same films subjected to heat treatment at 150° C. for one hour, weregently placed on a horizontal flat base to bring them into a concavestate thereon, and the state was visually observed without applying anyexternal force thereto

Results of determination and observation are shown in the followingTable 3. In Comparative Example 6, the film obtained by etching wasbroken, so that other items than adhesive strength (0.3 Kg/cm) could notbe determined, and curling of the laminate itself was observed.

TABLE 3 Comp. Determination/observation item Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex.9 Ex. 5 Tg (° C.) 312 330 311 321 304 329 CTE (ppm/° C.) 21 19 23 23 2518 Adhesive strength (Kg/cm) 1.2 1.5 1.1 1.2 1.0 1.2 Elastic modulus(GPa) 4.8 3.8 4.2 3.9 4.2 4.1 Tensile strength (MPa) 256 204 202 193 198239 Elongation at break (%) 47 30 38 32 48 42 Water absorbability (%)2.3 3.0 2.3 2.8 2.1 3.0 Percent shrinkage after etching MD direction (%)0.049 0.085 0.023 0.043 −0.013 0.069 TD direction (%) 0.060 0.109 0.0350.049 −0.011 0.076 Percent heat shrinkage MD direction (%) 0.039 0.081−0.022 0.016 −0.084 0.083 TD direction (%) 0.058 0.110 −0.019 0.035−0.062 0.090 Curling Laminate flat flat flat flat flat a little curledFilm obtained by etching flat flat flat flat flat a little curledHeat-treated film flat flat flat flat flat pencil state Note) “Pencilstate” means such a state that the film is curled and shrinked into abar-like form

INDUSTRIAL UTILITY

A metal laminate manufactured by laminating the present novel polyimidecopolymer to a metallic foil has a low curling susceptibility to causecurling, twisting, warping, etc. against temperature changes due to alow coefficient of linear thermal expansion of the polyimide copolymer,and also has satisfactory adhesiveness and thermal dimensional stabilityand a low water absorbability, and thus can be used as a suitable for aflexible, finely printed circuit board requiring a high dimensionalstability.

Furthermore, in the manufacture of a metal laminate by laminating apolyimide copolymer layer directly to a metallic foil withoutinterposing an adhesive layer therebetween, it is not necessary to forma plurality of polyimide copolymer layers as in the prior art. In thepresent invention, a metal laminate with desired properties can bemanufactured by a simple method, i.e. by forming a single polyimidecopolymer layer on a metallic foil.

1. A metal laminate manufactured by laminating a single layer of apolyimide copolymer to a metallic foil which metal laminate is subjectedto an etching process to remove a portion of the metallic foil saidetching process being conducted after the single layer of the polyimidecopolymer is laminated to the metallic foil, said polyimide copolymercomprising two kinds of tetracarboxylic acid dianhydrides consisting of(A) isopropylidenebis (4- phenyleneoxy-4-phthalic acid) dianhydride and(B) 3,3′,4,4′-biphenyltetracarboxylic acid dianbydride, and two or threekinds of diamines consisting of (C) 6-amino-2-(p-aminophenyl)benzimidazole and (D) at least one kind of diamines consisting ofbis(4-amninophenyl) ether (D₁) and phenylenedianhifle (D₂) and saidpolyimide copolymer being resistant to curling resulting from the metallaminate etching process so that the resulting etched metal laminate issubstantially curl-free.
 2. A metal laminate according to claim 1,wherein the copolymer has a film formability.
 3. A metal laminateaccording to claim 1, wherein the two kinds of tetracarboxylic aciddianhydrides are used in a proportion of component (A) to component (B)of 10-80 mol % to 90-20 mol % and the diamines are used in a proportionof component (C) to component (D₁) of not less than 60 mol. % to notmore than 40 mol. %.
 4. A metal laminate according to claim 1, whereinthe two kinds of tetracarboxylic acid dianhydrides are used in aproportion of component (A) to component (B) of 10-80 mol % to 90-20mol. %, and the diamines are used in a proportion of component (C) tocomponent (D₂) of not less than 20 mol. % to not more than 80 mol. %. 5.A metal laminate according to claim 1 for use as a flexible printedcircuit board.